1. Field of the Invention
The present invention relates to a method of automatically mounting a wheel to a wheel hub of a vehicle being conveyed along an assembly line, by detecting the center of the wheel hub, angular positions of hub bolts and a turning angle of the hub face.
2. Description of the Related Art
An automatic assembly system for mounting parts to a vehicle being conveyed on an assembly line by industrial robots has been progressing in the motor manufacturing industry. There, a method of acquiring information about images of wheel hubs, taken by cameras each having a charge-coupled device (CCD) has been employed in a wheel mounting robot for automatically mounting wheels to their corresponding wheel hubs. According to the method, the positions of hub bolts are detected by computation from the read image information.
However, in this type of detecting method, as it is difficult to locate the CCD cameras in the vicinity of the wheel hubs, the image of each wheel hub cannot be closed-up to the maximum on the screen. Therefore, the positions of the hub bolts cannot be detected with satisfactory accuracy.
On the other hand, a technical idea that a computational process is effected on read image to thereby improve the accuracy in the detection of respective positions of hub bolts, has been disclosed in Japanese Patent Application Laid-Open Publication No. 1-145288 entitled "A Method of Mounting Wheels To Vehicle", for example.
According to the above disclosure, information about images of respective wheel hubs is taken by cameras and information on a plurality of points recognized as hub bolts are detected or extracted from the images taken by the cameras. Then, recognized points identical in number to the hub bolts to be mounted are selected from the detected points. Further, a polygon formed by connecting the selected recognized points to each other by straight lines is compared with an array pattern of a predetermined polygon used as the standard. Thereafter, a combination of recognized points whose errors are held to the minimum is specified as the positions of the hub bolts.
In the conventional vehicle-wheel mounting method, however, often, when a hub face is not parallel with a reference face, the position of each hub bolt cannot be detected. When the number of hub bolts varies, i.e., when the kind of a vehicle to be assembled is changed, the method cannot rapidly meet such a situation.
Further, all the recognized points which have been detected, must be combined to produce polygons, and each of the produced polygons must be compared with the standard polygon. Therefore, the conventional wheel mounting method is time-consuming for their computation, and thus, has suffered from low productivity.